Low-frequency (<20 MHz) ultrasound is one of the most common imaging modalities in diagnostic medicine. The success of this modality can mostly be attributed to its ability to provide safe, reliable, real-time images of sub-surface tissue structures. Over the last 50 years, technological advancements in low-frequency ultrasound hardware and fabrication equipment have enabled an increasing level of sophistication in these systems.
In contrast, high-frequency ultrasound is a relatively new area of ultrasonic imaging that provides an order of magnitude improvement in image resolution compared with conventional low-frequency systems. Although these high-frequency systems can resolve tissue structures smaller than 50 microns in size, they are not routinely used in clinical practice. One of the barriers preventing their adoption and clinical utility is that the current systems are based on single-element geometrically-shaped transducers that have conventionally produced images with a limited depth-of-field, limited penetration depth, and relatively slow frame-rate.
In low-frequency ultrasound systems, drastic improvements in both frame-rate and depth-of-field have been achieved by replacing the single-element transducer with a transducer array and an electronic beamformer. The combination of a transducer array and an electronic beamformer allows the ultrasound energy to be electronically focused at a wide range of depths within the tissue at increased frame-rates. Consequently, there has been a great deal of interest in developing array-based systems for ultrasound frequencies greater than 20 MHz.
Unfortunately, fabricating high-frequency transducer arrays and associated beamformers is complicated by the increased ultrasound frequency. In particular, to produce a tightly collimated ultrasound beam, array elements with extremely small dimensions are needed and the digital sampling resolution of the electronic beamformer has to be greatly increased. Although some success in developing high-frequency transducer arrays has been recently reported, the intended applications for these arrays are somewhat limited. The transducer arrays are designed for use in general topical applications in which relatively large apertures and packaging are of no concern.
Improvements in high-frequency ultrasound imaging systems are therefore desirable.